1. Field of the Invention
The invention relates to an electrode structure, and in particular to an electrode structure of a fuel cell for power generation.
2. Description of the Related Art
In general, fuel cells having an anodic structure, a cathodic structure and an ionic exchange membrane generate power by converting chemical energy to electrical energy by electro-chemical reaction therebetween. In the process of electro-chemical reactions have the fuel oxidation and oxygen reduction. The fuel oxidation reaction of the anodic structure releases hydrogen ions, electrons and carbon dioxides. The oxygen reduction reaction of cathodic structure combines with anodic hydrogen ions, and the electrons releases water. In a conventional fuel cell, however, crossover of the liquid, colloidal, solid or gaseous organic fuel (e.g., alcohol, aldehyde or acid) from the anodic to cathodic structure is inevitable. Part of the fuel and water without reacting with the anodic catalyst directly pass through the ionic exchange membrane and reach the cathodic structure, resulting in a decrease of the catalytic reaction performance of the cathodic structure. A mixed potential is formed by the fuel on the cathodic structure produced oxidation reaction and with the nearby oxygen produced reduction reaction, thus reducing output voltage, output electrical power and fuel utilization of the fuel cell. Further, crossover of fuel results in the swell of the electrode adhesive between the cathodic electrode and the ionic exchange membrane, thus accelerating aging of the cathodic structure.
In FIG. 1A, a conventional anodic structure A1 of an organic fuel cell sequentially comprises a first substrate layer 111, a platinum alloy carbon support layer 112 and an ionic exchange membrane 2. The platinum alloy carbon support layer 112 disposed next to the ionic exchange membrane 2 comprises pluralistic carbon support particles 116, each covered by pluralistic platinum alloy particles 115 and appropriately polymers 118. The platinum alloy particles 115 catalyzed the liquid, gel, solid or gaseous organic fuel, e.g. alcohol, aldehyde or acid, of the anodic structures A1 to produce oxidation reaction. Because the platinum alloy particles 115 of the platinum alloy carbon support layer 112 are optimally distributed on the carbon support particles 116, the effective catalytic area and availability of the platinum alloy particles 115 increases. In FIG. 1B, a reference signal D1 represents a fuel path when the fuel travels through the platinum alloy carbon support layer 112. Due to the larger size of carbon support particles 116 and larger pore sizes therebetween, the fuel path D1 is relatively short. In FIG. 1C, a reference signal B1 represents a fuel concentration curve when the fuel travels through the anodic structure A1. The utilization of the platinum alloy particles 115 of the platinum alloy carbon support layer 112 is high and the fuel path D1 is relatively short, resulting in a smoothly declining fuel concentration curve B1.
In FIG. 2A, an anodic structure A2 sequentially comprises the first substrate layer 111 and a platinum alloy black layer 121 disposed next to the ionic exchange membrane 2. The platinum alloy black layer 121 comprises the platinum alloy particles 115 and appropriately polymers 118. Because the platinum alloy particles 115 tend to aggregate, the inside of the aggregated platinum alloy particles 115 cannot react with the fuel, and thus the effective catalytic area of the aggregated platinum alloy particles 115 is low, i.e., the availability of the platinum alloy particles 115 decreases. In FIG. 2B, a reference signal D2 represents a fuel path when the fuel travels through the platinum alloy black layer 121. Because the platinum alloy particles 115 are small and the pore sizes between are tortuously, the fuel path D2 of the fuel traveling through the platinum alloy black layer 121 is relatively long. In FIG. 2C, a reference signal B2 represents a fuel concentration curve when the fuel travels through the anodic structure A2. The utilization of the platinum alloy particles 115 of the platinum alloy carbon black layer 121 is low and the fuel path D2 is relatively long, resulting in a sharply declining fuel concentration curve B2.
FIG. 3 shows a discharge interval of the cathode and anode potentials of the fuel cell. A reference signal VA represents a theoretical potential of cathodic oxygen reduction, and a reference signal VB represents a practical potential of cathode discharge. The distance is between the theoretical potential VA and the practical potential VB of cathode discharge is an over-potential caused by cathodic oxygen reduction. A reference signal VC represents a practical potential of anode discharging, and a reference signal VD represents a theoretical potential of the fuel. The distance is between the practical potential VC of anode discharging and the theoretical potential VD of the fuel is an overpotential caused by fuel oxidation reaction. The distance between the cathodic practical potential VB and the anodic practical potential VC is an output voltage of the discharge fuel cell.
There is fuel oxidation reaction in the cathode, when the fuel of anodic structure permeates through the cathodic structure. The cathodic practical potential VB has gone down and the output voltage of the fuel cell has decreasing. It is understood that crossover of the fuel from the anodic structure throughout the cathodic structure is an unwanted situation.
The platinum alloy carbon support layer 112 of the anodic structures A1 or the platinum alloy black layer 121 of the anodic structure A2 must be thickened can solve the problems such as fuel crossover caused by the described liquid, gel, solid or gaseous organic fuel (e.g., alcohol, aldehyde or acid) The fuel path D1 is relatively short and the thickened platinum alloy carbon support layer 112 can avoid the corssover of the fuel, however, cracks form on the platinum alloy carbon support layer 112 and lowers the utilization of the catalyst adjacent to the ionic exchange membrane 2 when the platinum alloy carbon support layer 112 is too thick.
Note that the fuel path D2 of the anodic structure A2 is longer than the fuel path D1 of the anodic structures A1. If the platinum alloy black layer 121 of the anodic structure A2 is thickened, the product of fuel reaction, e.g. carbon dioxide, requires more time to travel through the fuel path D2, thus, the efficiency of the fuel cell decreases. In general, the cost of the platinum alloy carbon support layer 112 or the platinum alloy layer black 121 is substantially 70% of the total material of a fuel cell. Thus, the thickened platinum alloy carbon support layer 112 or the thickened platinum alloy black layer 121 increases the fuel cell material cost.
Based on the defects caused by the described fuel crossover of the liquid, gel, solid or gaseous organic fuel, e.g. alcohol, aldehyde or acid and the low catalyst utilization, the invention provides an electrode structure utilizing a small amount of catalyst in a catalytic layer to lower fuel crossover, consume the fuel in the anodic structure, avoid the fuel from the anode structure diffusion to the cathodic structure, increase the output voltage of the fuel cell.